A cation exchange membrane is one having a cation exchange group in itself to allow selective permeation of cations and is used in the field of electrodialysis, diffusion dialysis, fuel cells, or the like. Particularly, in the field of fuel cells, a cation exchange membrane is used widely for polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), redox flow batteries (RFB), or the like. An ion exchange membrane provided with lower electrical resistance, higher selective ion permeability, higher chemical stability and higher mechanical strength has higher utility.
Particular examples of the ion exchange membranes known to date include those developed by using non-fluoropolymers and polymers partially substituted with fluorine. Typical examples of such cation exchange membranes include those using polymers based on sulfonated poly(phenylene oxide), poly(phenylene sulfide), polysulfone, poly(para-phenylene), polyetherether ketone, polyimide, or the like. Most of commercially available cation exchange membranes include a fluoropolymer having a cation exchange group introduced thereto. Typical examples of such cation exchange membranes include Nafion available from DuPont, Dow membranes available from Dow Chemicals, Aciplex-S membranes available from Asahi Chemicals, and Flemion membranes available from Asahi glass. Particularly, Nafion™ includes a sulfonate group introduced to a polytetrafluoroethylene backbone, and has an ion conductivity of 0.1 S/cm under its saturated moisture content, high mechanical strength and chemical resistance. Thus, it has been used widely as an electrolyte separator in a vanadium redox flow battery.
Meanwhile, an ion exchange membrane used as a constituting element of a redox flow battery is a semi-permeable membrane which separates an electrolyte solution of a cathode from that of an anode while allowing selective permeation of cations or anions during oxidation/reduction. In addition, such an ion exchange membrane includes a main chain supporting the structure of the membrane and a function group having an ion exchange function. An ion exchange membrane is a main component that has an important effect upon the output, capacity, lifespan and cost of a redox flow battery. An ion exchange membrane including an organic material always exists in a state where it is immersed in an electrolyte solution, and thus shows higher possibility of undergoing degradation of its quality due to oxidation or the like, as compared to the other components. Therefore, the durability of such an ion exchange membrane becomes a main factor determining the lifespan of a redox flow battery. Particularly, it is problematic that the cathode and anode electrolyte solutions that are aqueous solutions damage the component, and thus causes broadening of the channel of an ion exchange membrane through which water passes, resulting in crossover of undesired ions. To prevent such a crossover phenomenon, it is known that fine inorganic particles are introduced into an organic ion exchange membrane so that the physical durability of the ion exchange membrane may be improved and the component may be prevented from being damaged. However, introduction of inorganic particles into an ion exchange membrane causes a drop in proportion of functional groups in the ion exchange membrane and a rapid decrease in ion conductivity, resulting in an increase in lifespan of a battery but degradation of quality.
Under these circumstances, the present inventors have invented an organic/inorganic hybrid ion exchange membrane by introducing inorganic particles to an ion exchange membrane, while introducing functional groups to the surface of the inorganic particles to prevent a decrease in ion conductivity and to improve physical properties.
Referring to the related art, Korean Laid-Open Patent No. 2013-0025582 discloses “Anion Exchange Membrane for Redox Flow Battery and Method for Manufacturing the Same”, Korean Laid-Open Patent No. 2010-0116888 discloses “Method for Manufacturing Separator for Vanadium Redox Flow Secondary Battery and Separator Obtained Thereby”, and Korean Laid-Open Patent No. 2013-0060159 discloses “Ion Exchange for Redox Flow Battery”. The above-mentioned patent documents disclose ion exchange membranes containing a reaction product of an organic compound or polymer with inorganic particles and thus having improved physical properties. However, such a reaction product is different from the organic/inorganic hybrid particles disclosed herein and the method for manufacturing the same is more complex than the method disclosed herein.